Liquefied Natural Regasification Plant

ABSTRACT

The invention relates to a plant for regasification of liquefied natural gas (GNL), comprising a liquefied gas storage reservoir ( 10 ) and a regasification device ( 12 ) for the GNL through which the natural gas and a heat transfer medium flow. According to the invention, the plant comprises a loop circuit ( 16 ) in which the heat transfer medium circulates in the form of a low-viscosity organic liquid with a low crystallisation point and the regasification device ( 12 ) comprises at least two exchangers ( 60, 62 ).

FIELD OF THE INVENTION

The present invention relates to a liquefied natural gas regasification plant and to a method used in same.

BACKGROUND OF THE INVENTION

Generally, when natural gas has to be transported from a production site to an exploitation site that are close together, this transport occurs through onshore or underwater pipelines. In this case, the natural gas is transported in gaseous form and it can be used as such at its point of destination.

However, when the two sites are too far away from one another or if the field configuration does not allow pipe laying, the gas is transported in liquefied form by land vehicle or by boat (generally LNG carriers) between the production site and the exploitation site. The natural gas is therefore liquefied in the vicinity of the production site during compression and cooling operations to a temperature of −160° C. The liquefied natural gas (LNG) is thereafter stored in suitable tanks, then transferred in liquid form to tanks for ground transportation or shipping to the exploitation site. Once on this site, this liquefied gas is unloaded into LNG storage tanks from which this gas can be regasified on demand and used either directly on the exploitation site or transported in gaseous form through pipelines to other exploitation sites.

Usually, in the case of LNG shipping, the liquefied gas is stored, then transported in the vicinity of the shore terminal in isothermal tanks of the LNG carrier. This liquefied gas is either regasified from the LNG carrier tanks, then transported in gaseous form through pipelines to exploitation sites, or sent in liquid form to tanks of the shore terminal where they are stored and regasified on demand.

Currently, to carry out the regasification operation, the gas in liquid form is pumped from the tank, then it flows through a set of heat exchangers acting as vaporizers or regasifiers. In order to provide heat exchange, seawater, possibly heated, is sent through this set of heat exchangers so that the calories present in this water are transmitted to the gas. This calories transmission allows the gas to be heated as it progresses through the set of exchangers, it progressively changes state and leaves the set of exchangers in gaseous form.

Such layouts however involve quite considerable drawbacks, as regards nature preservation as well as integrity of the exchangers.

In fact, the seawater that has passed through the heat exchangers is discharged at sea at a very low temperature, which causes degradation of the submarine flora and fauna. Besides, seawater is a corrosive agent towards all the metallic parts of the exchangers and it therefore requires closer maintenance of these exchangers. Furthermore, considering the fact that the LNG circulates in the exchangers at a very low temperature, the seawater has to flow through the exchangers with a high flow rate so as to prevent crystal formation, which requires large-size and expensive pumping facilities.

The present invention aims to overcome the aforementioned drawbacks by means of a regasification plant using a heat carrier allowing to respect the environment and that can be used far from shore terminals.

SUMMARY OF THE INVENTION

The present invention thus relates to a liquefied natural gas regasification plant comprising a tank for storing the gas in liquefied form and an LNG regasification device through which a heat carrier and the natural gas flow, characterized in that the plant comprises a loop circuit in which the heat carrier circulates in form of a low-viscosity and low-crystallization point organic fluid, and in that the regasification device comprises at least two exchangers.

The plant can comprise a heat carrier heating unit.

Advantageously, air can flow through the heating unit.

The heat carrier can have a crystallization temperature ranging between −90° C. and −150° C.

Preferably, the heat carrier can be an alcohol such as methanol, ethanol or propanol.

One of the exchangers can be co-current between the LNG and the heat carrier and the other exchanger can be counter-current.

The counter-current exchanger can be in two parts between which a phase separator is interposed.

At least the counter-current exchanger can be of brazed plate-fin exchanger type.

The heat carrier circulation circuit can comprise an additional heating exchanger.

The plant can comprise means for liquefying a hydrocarbon by calorific exchange with the heat carrier.

The hydrocarbon can be in gaseous form after being used for driving a turbine.

Advantageously, the hydrocarbon can be propane.

The plant can also comprise means for CO₂ trapping by the heat carrier.

Preferably, the heat carrier can be used as solvent of the CO₂.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying figures wherein:

FIG. 1 diagrammatically shows the LNG regasification plant according to the invention,

FIG. 2 is a partial sectional view of the heater used in the plant according to the invention,

FIG. 3 is a diagrammatic sectional view of the regasifier used in this plant,

FIG. 4 is a first variant of the regasification plant according to the invention,

FIG. 5 is another variant of the regasification plant according to the invention,

FIG. 6 shows an example of a particular use of the plant according to the invention, and

FIG. 7 shows another example of a use of the plant according to the invention.

DETAILED DESCRIPTION

FIG. 1 diagrammatically shows a liquefied natural gas (LNG) regasification plant comprising a storage tank 10 for storing the LNG at atmospheric pressure and at a temperature close to −160° C., a regasification device with a heat exchanger unit, or regasifier 12, through which a heat carrier and the LNG from the tank flow, and a heat carrier heating unit 14.

The heat carrier is an organic fluid whose crystallization point is close to that of the LNG and it has a viscosity that is low enough to be led to readily circulate in these pipes even at very low temperatures. Furthermore, this heat carrier remains in the liquid state under conditions of use at atmospheric pressure and at ambient temperature. Preferably, this heat carrier can be an alcohol or a hydrocarbon or one of their compounds. In the description hereafter, the organic fluid considered by way of example is methanol whose crystallization point is approximately −98° C., but it is also possible to use other alcohols such as ethanol (crystallization point: −114° C.) or propanol (crystallization point: −126° C.).

This plant comprises a heat carrier circulation loop 16 that, in the example shown, is a closed loop with a warm part and a cold part. This loop comprises a circulation pump 18, a line 20 for circulation of the heat carrier between the pump and regasifier 12, a circulation line 22 between the regasifier and heating unit 14, a return line 24 between this heating unit and the circulation pump, a heat carrier tank 26 being arranged on this return line. The plant also comprises an LNG suction pump 28 generally immersed in tank 10, a LNG circulation line 30 between this pump and a circulation pump 32, a line 34 bringing the LNG from this circulation pump to regasifier 12, and an outlet line 36 intended to carry the gas in gaseous form from the regasifier to any suitable means. A heating fluid 38 that is, in the example illustrated, outside air at ambient temperature also flows through the heating unit comprising a line 40 for discharge of the condensates from this air. Of course, this heating air can also come from any equipment present in the exploitation site, such as the fumes discharged by a gas turbine.

To achieve regasification, the LNG is pumped from tank 10 by pumps 28 and 32, then it circulates in lines 30 and 34 to be sent to regasifier 12. This gas circulates in the regasifier through which the methanol used as heat carrier also flows. The methanol present in tank 26 is therefore pumped by pump 18 and it is sent through line 20 to regasifier 12. In this regasifier, the calories present in the methanol are transmitted to the LNG and heat it so that the liquid phase of the LNG is changed into a gas phase by vaporization then, if necessary, it is overheated to reach a temperature close to the ambient temperature.

The temperature of the methanol at the inlet of regasifier 12 is about 20° C. and the temperature of the LNG circulating in line 34 is about −160° C. At the outlet of this regasifier, the natural gas is at a temperature close to 5° C. whereas the methanol reaches a temperature of about −70° C. at the outlet of this regasifier in line 22.

During exchange in the regasifier, the methanol is cooled to a temperature above its crystallization point, i.e. −70° C. for the example considered. The cold methanol is sent through line 22 to heating unit 14 so that the air circulating in this unit, whose temperature is higher than that of the cold methanol, exchanges its calories with this methanol to obtain a heated methanol in line 24 and, consequently, in tank 26.

The temperature of the methanol at the heating unit inlet is of the order of −70° C., whereas the air is fed into this heater at a temperature close to 30° C. After calorific exchange in this unit, the methanol is discharged at the unit outlet at a temperature close to 0° C., whereas the air leaves the unit at a temperature close to 5° C.

Thus, the warm part of loop 16 is made up of line 24, tank 26, pump 18 and line 20, whereas the cold part of this loop comprises line 22.

To achieve heating of the methanol at the regasifier outlet, and as illustrated in FIG. 2, heating unit 14 comprises a heat exchanger including a vertical shell 42 with an air inlet 44 and an air outlet 46 arranged at each end of this shell. This shell houses a set of vertical tubes 48 connected, at one end thereof, by an intake manifold 50 to an inlet 52 for the cold methanol coming from the regasifier and, at the other end thereof, by a discharge manifold 54 to an outlet 56 linked to line 24 leading to methanol tank 26. In this heat exchanger, the methanol flows in through inlet 52, enters intake manifold 50, circulates in all the vertical tubes 48 and ends in discharge manifold 54 prior to being discharged through outlet 56. Simultaneously, air, either at ambient temperature or heated by any known means, is fed into shell 42 through inlet 44, then it scavenges all the tubes and the manifolds. During scavenging, the calories contained in this air are transmitted to the methanol so as to heat it and to obtain a warm methanol at outlet 56. During this exchange, the water droplets contained in the air are condensed, then they fall through gravity at the bottom of shell 42 prior to being discharged in form of condensates through line 40. Tubes 48 can be coated with a hydrophobic material film (water shedding film) of polymethylsiloxane type to facilitate separation of the water droplets.

In connection with FIG. 3, the regasifier comprises a vertical shell 58 that contains at least two exchangers in which the gas and the methanol circulate, an upper exchanger 60 arranged in the upper part of the shell and a lower exchanger 62 arranged in the lower part of this shell. Preferably, these exchangers are in form of brazed plate-fin exchangers, advantageously made of aluminium. The upper exchanger, referred to as counter-current exchanger because the natural gas and the methanol circulate in opposite directions whereas the lower exchanger is referred to as co-current exchanger, the fluids circulating in the same direction. Thus, the lower exchanger comprises, on one of its sides and in the lower part thereof, a methanol inlet 64 connected to line 20 and an outlet 66 on one side of the exchanger. This lower exchanger also comprises an inlet 68, connected to LNG line 34, which is located in the lower part and on the side opposite the methanol inlet, and an outlet 70 located in the upper part of the exchanger. Thus, in lower exchanger 62, the methanol and LNG streams circulate in the same direction, i.e. from the bottom to the top of this exchanger. The skin temperature within this exchanger therefore remains above −100° C. and the exchange surfaces can be minimized. Methanol outlet 66 is connected by a line 72 to an inlet 74 of the upper exchanger that is located in the upper part and on one side of this exchanger. Similarly, natural gas outlet 70 is connected by a line 76 to a gas inlet 78 located in the lower part of this exchanger. The gas in vapour form is discharged through an outlet 80 located in the upper part of this exchanger whereas methanol outlet 82 is located in the lower part of this exchanger and connected to line 22 leading to the heating unit. This exchanger is thus referred to as counter-current exchanger because the gas and methanol streams circulate in opposite directions, the gas from the bottom to the top of the exchanger and the methanol from the top to the bottom of the exchanger.

In the variant shown by way of example in FIG. 4, regasifier 12 is divided into two distinct parts. Thus, co-current exchanger 62 comes in form of a shell-and-tube exchanger, and it comprises inlets 64, 68 and outlets 66, 70 for methanol and LNG. Outlets 66 and 70 are connected by lines 72, 76 to counter-current exchanger 60 that is a brazed plate-fin exchanger, advantageously made of aluminium, comprising inlets 74, 78 and outlets 82, 80 for methanol and natural gas.

Preferably, the shell-and-tube exchanger comprises a mechanical expansion joint 83 that absorbs all the dimensional variations of this exchanger as the LNG and the methanol flow therethrough.

In this variant, operation of the plant is the same as described in connection with FIGS. 1 to 3.

FIG. 5 shows a variant of the regasification plant illustrated in FIG. 4, which therefore comprises the same reference numbers for the common elements.

This variant differs in that regasification is carried out in several stages. Furthermore, counter-current exchanger 60 is in two parts 60A, 60B and a phase separator 84 is provided between these two parts of the exchanger.

The natural gas leaving co-current shell-and-tube exchanger 62 through outlet 70 is preheated to its boiling point corresponding to the pressure in separator 84. This heated liquid natural gas flows through lower part 60A of counter-current exchanger 60 to achieve a phase conversion through vaporization. This converted natural gas is sent through a line 86 to separator 84 where separation of the natural gas in gaseous form occurs in upper part 88 of this separator with a lower composition, molecular weight and calorific value, and in liquid form in lower part 90 of this separator. The natural gas in vapour form present in the separator is then sent, through a line 92, from this separator to the inlet of part 60B of exchanger 60 where it undergoes, by exchange with the methanol circulating therein, a temperature rise until it reaches outlet 80. The liquid phase, whose molecular weight and calorific value are higher than that of the vapour, is extracted by a pump 94 connected to this separator by a line 96. The liquid phase leaving pump 94 is sent through a line 98 to any storage means prior to being treated. Advantageously, it is possible to control the composition and the calorific value of the natural gas in gaseous form in line 92 before it enters exchanger 60 by injecting a predetermined amount of liquid coming from the separator through a line 98A starting after pump 94 on line 98 and ending on line 92.

In this configuration, the temperature of the natural gas at the regasifier outlet is of the order of 0° C., the temperature of the methanol is about −70° C.

In addition, it is possible to heat the methanol at the outlet of pump 18 by placing on line 20 a heat exchanger 100 for exchange between the methanol and a warm fluid that is commonly used in or close to this regasification plant, such as warm water from trickle towers.

As described above, the methanol at the regasifier outlet is at low temperature, of the order of −70° C., and it has to be heated to provide conversion of the LNG to gas phase in the regasifier. It is therefore possible to take advantage of the presence on the site of an electric power plant with a combined cycle gas turbine as illustrated in FIG. 6. In this case, plant 102 is supplied with air through a channel 104 and with natural gas through a channel 106; this channel can be a bypass of line 36 described above. Combustion of the air-natural gas mixture within the turbine generates, after recovery of the calories generated (HRSG), at outlet 108, fumes with temperatures of the order of 130° C. As shown in FIG. 6, these fumes are fed through an inlet 110 into a heat exchanger assembly 112, divided into at least three parts 112A, 112B, 112C, and leave through a discharge line 114 prior to being sent through a line 116 to any suitable means, such as a chimney. A phase-change fluid such as propane also flows through the heat exchanger assembly and circulates in a closed loop 118. This loop comprises a liquid propane tank 120, a circulation pump 122 connected to the tank by a line 124 and a propane phase separator 126 connected to the pump by a line 128E carrying the liquid propane to part 112A of the heat exchanger assembly and a line 128S carrying the propane, preheated to its boiling point, into this separator. Two lines start from this separator: a line 130, referred to as liquid line, wherein the liquid contained in the separator is brought to part 112B of the heat exchanger assembly, then flows therethrough and flows back in gaseous form into separator 126, and a line 132, referred to as gas line, that carries the gas phase of the propane contained in the separator to part 112C of the heat exchanger assembly so as to overheat this propane gas. A line 134 brings the propane in pressurized gaseous form to an expansion turbine 136 coupled in rotation to any energy producing means such as an alternator 138. At the outlet of the expansion turbine, the propane gas is sent through a line 140 to a heat exchanger 142, referred to as condenser, in order to cool this propane gas and thus to cause a phase change so as to obtain a liquid phase before it flows back to tank 120 through a line 144. To cool the propane, the methanol circulating in line 22, as described above, flows through condenser 142 and, at the outlet of this condenser, the methanol is at a higher temperature than at the inlet because it has collected the calories contained in the propane in gas phase.

During operation, the propane in liquid form is pumped from tank 120 and flows through part 112A of exchanger assembly 112. The preheated propane in liquid form is thereafter sent to separator 126. The liquid phase extracted from this separator flows through part 112B of assembly 112 and flows back in nearly gaseous form into the separator for separation of the liquid phase and the gas phase of the propane. The gas phase contained in this separator is also extracted to flow through part 112C of exchanger assembly 112 to be totally converted to gas phase and overheated if necessary. The propane in gaseous form flows through turbine 136 that it drives into rotation, said turbine driving alternator 138 into rotation. At the turbine outlet, the propane in gaseous form flows through condenser 142 where it undergoes phase change and changes to the liquid phase by exchanging its calories with the cold methanol that also circulates in this condenser. At the outlet of this condenser, the liquid propane is stored in tank 120.

The treating group as diagrammatically shown in FIG. 7 shows a potential use of the LNG regasification plant with a methanol loop for collecting and liquefying the CO₂ contained in discharges, such as the fumes from gas turbines.

This configuration involves an LNG regasification plant 146, a CO₂ collection/separation plant 148, a methanol heating unit 149 and a CO₂ liquefaction unit 150.

Regasification plant 146, as already described in connection with the previous figures, comprises a regasifier 12 through which flow warm methanol circulating in a loop 152 and LNG coming from line 34.

CO₂ collection/separation unit 148 comprises an absorption column 154 containing transfer elements 156 with an inlet 158 for the methanol from the regasifier, an inlet 160 for a CO₂— containing gaseous fluid, an outlet 162 for a CO₂-freed gaseous fluid and an outlet 164 for a mixture of ethanol and CO₂. This CO₂ collection/separation unit also comprises a flash drum 166 with an inlet for the methanol-CO₂ mixture, an outlet 168 for the CO₂ in gaseous form and an outlet 170 for the methanol freed of a very large part of the CO₂.

Heating unit 149 comprises elements identical to those already described in connection with FIGS. 1 and 2, i.e. a heater through which flow the methanol coming, in the example illustrated in FIG. 7, from outlet 170 of drum 166 and a heating fluid 38 that can be outside air at ambient temperature. This exchanger also comprises a discharge line 40 for the condensates from this outside air. This unit finally comprises a heat exchanger 174 allowing to heat the methanol after its passage through the heater through an outlet 172 and a flash drum 175 allowing to separate the methanol in liquid form, which is then sent through a line 176 to the methanol loop, and the CO₂ in gaseous form that joins, through a line 178, a line 180 also connecting CO₂ line 168 of flash drum 166.

Liquefaction unit 150 comprises a condenser 181 whose specific feature is to use an intermediate fluid, such as ethane, to take part in the liquefaction of the CO₂ and in heating the natural gas in vapour form.

This condenser comprises an enclosure 182 that contains at least two condenser parts 184 and 186, each one counter-current and preferably in form of brazed aluminium plates and fins, in which circulate the CO₂ in vapour form and the ethane for the first one and the LNG and the ethane for the second. Lower condenser 184 is arranged in the lower part of the enclosure and comprises, on one side thereof and in the upper part of this condenser, a CO₂ inlet 188 connected to line 180 and a liquid CO₂ outlet 190 on the lower part of the condenser. Upper condenser 186 comprises an LNG inlet 192, connected to LNG line 34, that is arranged in the lower part of this condenser and an outlet 194 arranged in the upper part of the exchanger. A closed ethane loop 196 allows the ethane to circulate between the two exchangers. More precisely, the vapour ethane is fed into upper ethane condenser 186 through an inlet 198 located in the upper part of the condenser, flows through this condenser and ends at a liquid ethane outlet 200 arranged in the lower part of this condenser, is brought through a line 202 to a liquid ethane inlet 204 located in the lower part of the lower CO₂ condenser, flows through the lower condenser and reaches an outlet 206 in the upper part of this condenser, and eventually reaches inlet 198 through a line 208.

During operation of the treating group described above, the LNG substantially follows the same regime as described in connection with FIG. 1, except that a bypass of LNG line 34 reaches inlet 192 of CO₂ liquefaction unit 150 and runs through upper condenser 186, then through outlet 194 to join line 36.

At the regasifier outlet, the methanol is sent through inlet 158 to column 156 that also receives a fluid containing a substantial part of CO₂, of the order of 12%, through inlet 160. After treatment in this column, the CO₂ is collected by the methanol, and a mixture of methanol and of dissolved CO₂ is discharged through outlet 164. The CO₂-freed fluid is discharged through outlet 162 to any suitable means. The mixture of CO₂ and of methanol undergoes separation in flash drum 166 from which the CO₂ in vapour phase is discharged through outlet 168 to line 180 and the methanol in liquid phase from outlet 170 is heated in the heating unit by passing successively through the heater and exchanger 174. At the outlet of exchanger 174, the residual CO₂ contained in the methanol is again separated from this methanol in flash drum 175. During this separation, the CO₂ is discharged through outlet 178 to join line 180 connected to outlet 168 and the CO₂-freed methanol joins, through outlet 176, pump 18 of the methanol loop. The CO₂ in vapour phase is liquefied in lower condenser 184 where it exchanges its calories with the ethane that circulates in a loop between the two condensers. After this exchange, the CO₂ is in liquid form at outlet 190 and it can be sent to a storage tank from where it can be removed to be possibly sequestered in underground reservoirs.

The present invention is not limited to the embodiment examples described and it encompasses any variant and equivalent. 

1) A liquefied natural gas (LNG) regasification plant comprising a tank for storing the gas in liquefied form and an LNG regasification device through which a heat carrier and the natural gas flow, characterized in that the plant comprises a loop circuit in which the heat carrier circulates in form of a low-viscosity and low-crystallization point organic fluid, and in that the regasification device comprises at least two exchangers. 2) A regasification plant as claimed in claim 1, characterized in that it comprises a heat carrier heating unit. 3) A regasification plant as claimed in claim 2, characterized in that air flows through the heating unit. 4) A regasification plant as claimed in claim 1, characterized in that the heat carrier has a crystallization temperature ranging between −90° C. and −150° C. 5) A regasification plant as claimed in claim 1, characterized in that the heat carrier is an alcohol such as methanol, ethanol or propanol. 6) A regasification plant as claimed in claim 1, characterized in that one of the exchangers is co-current between the LNG and the heat carrier, and in that another of the exchangers is counter-current. 7) A regasification plant as claimed in claim 6, characterized in that the counter-current exchanger is in two parts between which a phase separator is interposed. 8) A regasification plant as claimed in claim 6, characterized in that at least the counter-current exchanger is of brazed plate-fin exchanger type. 9) A regasification plant as claimed in claim 1, characterized in that the heat carrier circulation circuit comprises an additional heating exchangers. 10) A regasification plant as claimed in claim 1, characterized in that it comprises means for liquefying a hydrocarbon by calorific exchange with the heat carrier. 11) A regasification plant as claimed in claim 10, characterized in that the hydrocarbon is in gaseous form after being used for driving a turbine. 12) A regasification plant as claimed in claim 10, characterized in that the hydrocarbon is propane. 13) A regasification plant as claimed in claim 1, characterized in that it comprises means for CO₂ trapping by the heat carrier. 14) A regasification plant as claimed in claim 13, characterized in that the heat carrier is used as solvent of the CO₂. 